Clarence Aaron described his time in federal prison serving life without parole for a first-time nonviolent drug conviction as "a walking death sentence on" his head. Before President Barack Obama commuted his sentence last month, Aaron faced a longer sentence than some fellow inmates with multiple felonies -- even murder.

Aaron was hardly the only nonviolent offender sentenced to life without parole. Last year, the American Civil Liberties Union reported that more than 2,000 federal inmates were serving life without parole for nonviolent offenses.

How can a nonviolent first-time offender be sentenced to life without parole?

Their biggest mistake, after their crimes, was not pleading guilty. "Prosecutors give drug defendants a so-called choice. In the most egregious cases, the choice can be to plead guilty to 10 years or risk life without parole by going to trial," Human Rights Watch adviser Jamie Fellner says. She has a term for it: "the trial penalty," which can deliver longer time than drug dealing itself.

When a case goes to trial, the prosecutor gets to pick the sentence by choosing the amount of drugs for which a defendant is charged. A federal judge had to sentence Aaron to life because he was convicted of conspiracy to traffic 23 kilograms of crack and powder cocaine. The crazy part is that prosecutors got to 23 kilograms by charging Aaron for a 9-kilogram deal that did happen and a 15-kilogram deal that did not.

The career dealers knew how to game the system. Their sentences were reduced because they testified against Aaron.

The National Association of Assistant United States Attorneys is fighting back against the bad press on federal mandatory minimums. The system, prosecutors argue, protects the public from the old days when a defendant's sentence depended on which judge heard the case. Now the guilty know that some prison time is unavoidable.

Georgetown University Law Center professor Bill Otis, a former federal prosecutor, reminded me that every system, every country, has its sentencing anomalies. The federal system at least has four checks -- plea bargains, a "safety valve" that allows judges to reduce the punishment for some low-level defendants, defendants' ability to reduce their penalty by testifying against others, and the presidential pardon power that commuted Aaron's life sentence.

I should add that Otis doesn't see drug dealing as nonviolent, not when an addict could die of an overdose.

I would expect to agree with everything Otis said. No system is perfect. Plea bargains benefit both law enforcement and defendants. The federal "safety valve" gives judges some discretion. Guilty parties can reduce their sentences by cooperating with authorities. If all else fails, there is the presidential pardon.

Except: The minimum sentence for a nonviolent offense should not be life without parole, basically the same as the maximum sentence.

Federal prosecutors want the public to trust them even when they do nothing to curb their own excesses. No reasonable person would assert that there is a public interest in putting young, low-level, nonviolent offenders, with or without prior convictions, behind bars until they die.

If the Department of Justice had demonstrated any kind of commitment to fixing its excesses, the public could trust prosecutors. Instead, the prosecutorial-industrial complex has circled the wagons and actually defends these medieval punishments.

Florida's Stephanie George was one of the other seven recipients of a presidential commutation last month. Like Aaron, who told me he was guilty of "a big crime," George made some huge mistakes. In 1996, authorities searched her house and found $14,000 and 500 grams of her boyfriend's cocaine stash in her attic. Because George had two prior felonies -- for selling $40 and $120 worth of crack, which were counted as separate felonies -- this 26-year-old woman was sentenced to life without parole.

When The Washington Post praised Obama for showing mercy toward a mother serving "a life sentence for stashing her boyfriend's drugs," the NAAUSA took offense. President Robert Gay Guthrie chided the editorial board for not recognizing that George's "prior convictions caused her to be considered a 'career criminal' with a criminal history at the highest level under the federal sentencing guidelines. Federal law, as mandated by Congress, caused her drug distribution conviction, her third, to trigger a life sentence."